Regarding the movement between members, I'm more inclined to think that the chincy screws chosen were the problem.

Right now I'm using 2 bungee cords.

Some friends at work directed me to an auto spring shop.  Over the phone, they indicated that they could bend threaded rod to my specifications.  I'm going to go over the frame, through the 2 slots on each side of the tray.

Incidentally, I noticed that's the same way the spare tire holder is attached to the rear bumper.